Memory cells of a nonvolatile semiconductor memory device can perform electrical rewriting, for example, by transferring charges between a charge storage layer, such as a floating gate or a dielectric film including a charge trap, and a semiconductor substrate. In the memory cell, transfer of charges is performed by applying a high electric field to a tunnel dielectric film between the charge storage layer and the substrate when writing data “0”. Moreover, in order to prevent erroneous writing to a selected cell when writing data “1” to the selected cell, a high electric field is prevented from being applied to the tunnel dielectric film of the selected cell by boosting the channel of the selected cell.
At this time, when memory cells on which writing has been performed increases, a boost coupling ratio decreases, so that erroneous writing is easy to occur. Therefore, in order to stabilize a boost level, a method called channel isolation of electrically isolating memory cells, on which has been written, when boosting a channel is used in some cases.